1. Field of the Invention
The present invention relates to an improved method and apparatus for an injecting and mold clamping control of a blow molding machine, and in particular, relates to a control method and apparatus for use in producing blow molded hollow articles of plastic materials such as engineering plastics, using an injection machine.
2. Description of the Related Art
A blow molding process is a well known method of producing hollow plastic articles, and according to a prior art, a blow molding machine comprises an injection machine part, called an "injection machine", but having a die defining a downwardly extending annular melt passage, preferably in a combination with a pre-blowing means.
The injection machine part is provided for plasticizing a plastic material and injecting the plasticized material into the atmosphere through the die, to thereby produce a hollow parison of the material ejected and sagging from the die. Preferably the ejected parison is subjected to a pre-blowing of air into the parison.
The blow molding machine further comprises a mold machine part comprising a pair of mold halves having parting faces and forming the mold. The mold halves define a mold cavity when the mold is closed. The mold machine part further comprises an air blowing means and a mold clamping device which is actuated to move the mold halves toward each other, to thereby clamp the mold while the parison is between the mold halves. The parison is subjected to the mold clamping while air is blown into a main portion of the parison enclosed by the cavity, to thereby expand the parison and urge same against the cavity, while the mold is cooled, to thereby produce a molded article formed by the parison in the cavity. The parison is separated from the die after the mold clamping, and before the next run begins, and thus the mold receives a fresh parison having a lower free end at every run.
In this type of blow molding machine, when the parison ejected and sagging from the die is gripped between the mold halves, the longer distance by which the free parison end protrudes out of the mold, the greater the amount of burrs produced, whereby the production yield is deteriorated. Conversely, if the mold halves are clamped before the free parison end has sufficiently sagged downward, a defective article is produced. Therefore, in the past, the practice has been to perform an adjustment such that, when the mold clamping is completed, the free parison end does not protrude too far outside the mold, i.e., only to an extent such that a suitable amount of burrs is produced.
The most general method used in the past for setting the mold clamping timing was, as shown in FIG. 4, to start the mold clamping after the end of the injection. The injection or ejection was ended after the output of an injection command (step 100). The end is confirmed by detecting the position of the plunger, etc., of the injection machine part (step 110). A previously set clamping timer was started to count the time (step 120), it was determined if the count had reached a predetermined value (step 130), and a clamping starting command was output (step 140). Usually, the timing timer was set to "0", and thus the mold clamping was started immediately after the end of the injection.
As another method, the injection and mold clamping were actuated in parallel. In this method, the clamping timing timer was started simultaneously with the injection command, and the mold clamping was started upon detecting that the count of the timer had reached a preset value (several seconds shorter than the 10 to 20 seconds injection time).
In the above-mentioned conventional method, however, in any case use was made of a timer which counted based on an input of the ejection signal of the parison or the injection signal of the plasticized material, and as the mold clamping was started when the time ran out, the following problems arose:
That is, recently a greater use has been made of engineering plastics, due to their expanded applications. In this case, in general, the speed of sag of the parison ejected from the die is high, and further, the solidifying speed is high, and thus a high speed injection is performed.
With a high speed injection, however, there is a large error in the position of the parison after a certain amount of time has elapsed from the start of the injection. If starting the mold clamping by the count of timer from the start of injection, in many cases the timing becomes off, and as a result, the mold is clamped too fast with respect to the position of the free parison end or conversely is clamped too slow, so there was a problem in that a satisfactory molding was not possible. That is, in the conventional method, the system counted the clamping timing by a timer after either the start of injection or the end of injection. The injection end occurs when the plunger of the injecting machine part is stopped to advance upon the free parison end reaching a position a predetermined vertical distance away from the lower end of the mold cavity toward the lower end of the mold. In this connection, if the injection speed was high or if use was made of an engineering plastic or other resin which easily sags downward, and the speed of sag of the parison was high, then by just clamping the mold according to the count of the timer without regard to the actual parison position or state, it was not possible to clamp the mold accurately in accordance with the parison position. This caused an excessive production of burrs or defective blow moldings due to an insufficient charging. Further, the timing of the start of the mold clamping was difficult to determine, so the cycle time of the blow molding per se became longer.
On the other hand, in a parison ejected and sagging from the die, there is a draw down phenomenon due to the weight of the parison itself. Further, the larger this draw down became, the larger the shape of the article, and this becomes even greater as the parison moves downward. Due to this, very complicated work was required for setting the timer. In actuality, it was necessary to set the mold clamping and injecting timings by trial and error after many attempts.